Interpretive Summary: Reducing weed seed inputs to the soil seedbank is an important goal for integrated weed management, yet most weed management tactics are targeted at the seedling stage. Development of effective strategies for weed seedbank management will be aided by a better understanding of the ecological factors regulating weed seed predation rates in agricultural fields. Predation rates of giant foxtail, velvetleaf and giant ragweed by invertebrate and vertebrate seed predators were measured within a corn crop in Illinois. Weed seed rain of the three species, activity-density of invertebrate seed predators and weather data were measured concurrently. Structural equation models indicated that weed seed supply and seed demand by predators have much greater explanatory power for variation in seed predation rates than do weather variables or seed demise due to other factors than predation. A better understanding of how predators forage, and under which conditions they become sated, will help guide agroecosystem design to increase weed seed predation.

Technical Abstract:
Weed seed predation is an important ecosystem service supporting weed management in low-external-input agroecosystems. Current knowledge of weed seed predation in arable systems focuses on biotic mechanisms, with less understood about the relative impact of abiotic variables on this process. In order to quantify relative contribution of abiotic and biotic variables to weed seed predation rates, a field study was made within a maize crop in central Illinois, USA, in 2005 and 2006. From late July through mid-October, weekly measurements were made of Abutilon theophrasti, Ambrosia trifida, and Setaria faberi seed removal rates by invertebrate and vertebrate granivores, and seed losses due to abiotic processes. Seed rain of these species was measured in contiguous plots. Concurrent measurements of temperature, precipitation, wind speed, thermal units and invertebrate activity-density were also made in the same location. Structural equation models of weed seed predation by invertebrates and vertebrates indicated that the relationship between seed supply and predator seed demand drove much of the variation in seed predation rates. When latent variables for abiotic seed losses and climatic variables associated with winter's approach were included in these models, their explanatory power and parsimony both declined. Therefore, the focus by many studies on biotic mechanisms in weed seed predation appears to be justified. A mechanistic understanding of predator satiation will help guide agroecosystem design to increase weed seed destruction by granivores.